They are known gaskets of the above type, suitable to be used in working vehicles, such as tractors or earth-moving machines, under extremely severe operating conditions, characterized for example by:                intermittent operation of the motor, i.e. with continuous passages from a start condition to a stop condition and vice versa;        continuous back and forth movements of the vehicles, with angular movements in opposite directions of the rotating members on which the aforesaid gaskets are applied;        partial immersion of the vehicles in mud;        adverse weather conditions; and        relatively high circumferential speed of the rotating members on which the aforesaid gaskets are applied.        
The gaskets of known type substantially comprise:                a support ring made of a metallic material, having a annular axial fixing portion, adapted to be connected in use to the fixed member;        a first ring of elastomeric material with a metallic inner core, radially more internal than the fixing portion of the support ring fixed to the fixing portion in a position adjacent to the first ambient and adapted to cooperate in use with the movable member to counter the passage of lubricant from the first ambient to the second ambient and the passage of external contaminants from the second ambient to the first ambient during the roto-translatory motion of the movable member;        a second sealing ring made of elastomeric material, carried by the support ring in a position adjacent to the second ambient and adapted to cooperate in use with the movable member to counter the passage of external contaminants from the second ambient to the first sealing ring; and        an annular wiper element, made of polyurethane or elastomeric material, axially interposed between the first and the second sealing ring and having an annular wiping lip adapted to cooperate in use with the movable member to counter the passage of external contaminants towards the first sealing ring.        
The axial movement of the movable member in opposite directions requires the use, on the first sealing ring, of two barriers represented by two annular sealing lips, axially spaced from each other, in order to provide an adequate sealing against external contaminants. In fact, possible wear areas on the outer surface of the movable member, such as incisions, grooves, etc., might become a receptacle for external contaminants which might pass, with the axial movement of the movable member, into the first ambient if only a single barrier were provided on the first sealing ring.
In order to be effective, the two sealing lips of the first sealing ring must necessarily be arranged at an axial distance greater than the maximum axial displacement allowed to the movable member during operation. In this way, in fact, any wear area located upstream of the two sealing lips before the axial movement of the movable member towards the first ambient might not reach the first ambient.
The described solutions, although being functionally effective in many applications, do not allow a large spacing between the two sealing lips of the first sealing ring, especially in the case of gaskets having reduced radial sections. In these cases, in fact, the first sealing ring cannot overhangingly support two sealing lips if they are spaced beyond a certain axial distance.